In conventional dry-type copiers, a statically charged latent image formed on a sensitizing drum to represent letters or numbers is converted into a toner image, which is then transferred onto a sheet of paper supplied from a paper feeding cassette, and the toner image transferred onto the paper surface is pressed and heated with a hot fixing roller to fix the image to the paper, thus inseparably fusing the toner image and the paper fibers together.
In order to discharge the paper sheet now carrying the image without getting caught by the fixing roller, the end of paper is scooped up by stripping fingers with their tips pressed tightly against the fixing roller.
Such stripping fingers are required to have a small frictional resistance so that they will not damage the outer peripheral surface of the roller, and to have a sufficient mechanical strength and high-temperature rigidity. Also their edges, especially their edge tips have to be shaped with high accuracy. Further, it is required that toner will not stick to them.
Many of recent copiers are actually not simple copiers but what are called intelligent copiers having high-resolution image processing and editing functions and facsimile function and equipped with input/output devices for use with other office automation machines. Such multi-functioned, complex, systematized copiers are required to operate at higher speed and have a higher reliability and longer life than ordinary copiers.
Thus, high processing speed is an essential requirement for recent copiers. The higher the processing speed, the higher is the heating temperature of the fixing roller. Thus, the stripping fingers have to have a still higher heat resistance. Further, such fingers will be exposed to high temperature for an extremely long time in order to keep the copier turned on so that it can be used at any time. Thus, the stripping fingers are required to have a good heat fatigue resistance.
Further, the stripping fingers are required to follow various operating conditions in multi-functioned copiers. Systematized copiers may be connected with devices which are used in life-or-death situations. High stripping reliability is required for the stripping fingers of such copiers. Namely, the tips of such fingers have to have a high heat load resistance sufficient to endure even in an accident such as paper clogging. Also, their tips have to be shaped so that reliable separation is assured even if the copier is used continuously for a long time.
Conventional stripping fingers are made of polyimide, polyamideimide, polyphenylenesulfide, polyetherketone, polyethersulfone or polyetherimide.
Of these materials, moldings of polyethersulfone or polyetherimide having ordinary heat resistance have a glass transition point of about 220.degree. C. and are amorphous. Since they soften at a temperature above the glass transition point, their heat resistance is too low for the stripping fingers used in high-speed copiers (250.degree. C. or more).
Some resins such as polyethersulfone and polyetherimide have a glass transition point of 250.degree. C. or more. But their lubricity and wear resistance are not good. This may lead to increased torque at the roller driving unit or poor separation. Even if a fluororesin coating is provided, the frictional surface in contact with the roller will wear with long use, so that friction will occur between the substrates of the stripping fingers and the roller.
Thus, poor lubricity and low wear resistance of the substrates lead to shorter life and lower reliability. Moldings of such resins as polyphenylenesulfide and polyetherketone have a glass transition point of less than 250.degree. C. But since they are crystalline resins, they can be reinforced by adding heat-resistant fibers such as glass fiber, potassium titanate fiber, carbon fiber or these fibers plus inorganic powdery fillers such as mica and talc, so that their heat resistance can be increased remarkably. But these materials have problems that the roller can be damaged and that if the reinforcing materials are not properly filled at the edges or tips of the stripping fingers, their resistance to heat deflection deteriorates markedly.
Of the polyimide resins, thermosetting polyimide resins, which can form a three-dimensional network, are brittle and thus require reinforcement with filling materials as with the above-mentioned polyphenylenesulfide resin.
Stripping fingers molded of polyamideimide resin have a heat resistance of 250.degree. C. or more even if reinforcing materials are not used. But they have a problem that their heat resistance deteriorates if they absorb water or moisture. If they absorb a relatively large amount of water, the heat resistance will deteriorate markedly. More specifically, if the molded article is heated at a rapid rate after absorbing water, the water content in it turns into high-pressure steam. It is well-known that if this happens in a molded article larger than a certain size, e.g. a sheet 127 mm long, 12.7 mm wide and 3.2 mm thick, the thickness increases 2.5 microns or more and the lowest temperature at which the surface swelling or foaming happens (what is called the thermal shock temperature) decreases markedly. The heat resistance of an article having a heat resistance of about 280.degree. C. in an absolute dry state will reduce to about 210.degree. C. if it absorbs a large amount of water.
There are known polyimide resins which are thermoplastic polyimides having a very large molecular weight. Such polyimides are commercially available from Du Pont under the trademarks Kapton and Vespel (Registered Trademarks). Although these resins have a high heat resistance, they are not practical because they cannot be made by melt molding such as injection molding.
Other potential candidates are aromatic polyesters, particularly liquid crystal polyesters which are melt moldable and show anisotropy at molten state (e.g. ones disclosed in Japanese Patent Publication 47-47870). This resin shows an orientation peculiar to liquid crystals, and shows self-reinforcement itself. Thus, its own heat deflection resistance is high. It is possible to improve the heat deflection resistance with smaller amounts of reinforcing materials such as inorganic heat-resistant fibrous fillers or powdery fillers. Further, since this material can be reinforced using fibers which are less likely to damage the counter material though their reinforcing effect is low compared with potassium titanate whiskers, attack on the counter material is less severe and brittleness due to oxygen crosslinking, which happens with polyphenylenesulfide resin, scarcely occurs, and heat aging resistance is also good.
Further, there will be no deterioration in the thermal shock temperature due to water absorption, which happens with polyamideimide resin moldings. Thus, those materials disclosed in Japanese Unexamined Patent Publications 62-245274 and 63-74084 have been used heretofore as materials for the stripping fingers. But they are not satisfactory in terms of reliability and longevity.
The surface temperature of the fixing roller in a copier is 150.degree. C. or higher in general and most typically in the range of 170.degree. C.-250.degree. C. Thus, if the finger tips are subjected to an inordinarily large load due to paper clogging or the like, they may creep under high-temperature load. Further, since the self-reinforcement is provided by the liquid crystals, which are rather large units, if they are subjected to stress repeatedly at high temperature, these units tend to collapse, causing a sharp deterioration in the physical properties such as flexural modulus. In other words, the heat fatigue resistance is poor.
One reinforcing material which can improve the high-temperature rigidity, heat fatigue resistance and heat load resistance and which is less likely to damage the roller material is potassium titanate. But its reinforcing effect and the degree of improvements are small. More importantly, a composition of liquid crystal polyester and potassium titanate whiskers is partially gelatinized when molded into stripping fingers by melting. This may lead to the formation of "blisters" on the surfaces of the fingers. If such blisters are present on the surface which contacts the roller, it would become impossible to strip paper sheets from the roller.
Further, the degree of self-reinforcement of the liquid crystal polyester due to its peculiar orientation varies widely. If it is small, its heat distortion temperature will be too low to be acceptable as stripping fingers. Further, if stripping fingers are molded of a liquid crystal polymer, the radius of curvature at their tips tends to be too small compared with those molded of a polyamideimide resin. Some of them will have even less than 10-micron sharp edges. Even if a stripping finger with a favorable radius of curvature at its edge (10-50 microns) is obtained by molding, its edge may be too sharp due to scratches formed on the mold by fillers or the like. Such a finger may suffer heat deflection as a result of reduced high-temperature rigidity. As a result, paper stripping may become difficult or the roller outer surface may be damaged.
On the other hand, numerous proposals have been made to improve the non-stick property of the stripping fingers with respect to toner. For example, it was proposed to form on a stripping finger a coating of fluororesin or fluorinated polyether polymer or to incorporate a non-stick property modifier such as a fluororesin in the material.
One conventional method which aims specifically to improve the non-stick property with respect to toner is to heat tetrafluoroethylene-perfluoroalkylvinylether copolymer (hereinafter abbreviated to PFA) above its melting point to fuse it to the stripping fingers. Since this technique does not use a binder resin (such as epoxy resin, polyimide resin or polyamideimide resin), which is used ordinarily in other techniques, the surface of the coating material solely consists of PFA resin. Thus, its non-stick property is excellent.
But in order to firmly bond the PFA film to the stripping fingers so that the PFA can exhibit its inherent excellent non-stick property, it has to be heated to 330.degree. C. or more. Very few resins can withstand such high temperatures. Even a stripping finger made of a liquid crystal polyester may deflect, shrink or develop blisters on the surface during the heat melting step.
As described above, there has been no stripping finger which has an excellent heat deflection resistance, heat aging resistance, thermal shock resistance, heat fatigue resistance and heat load resistance, which attacks the counter roller less severely, and which has an excellent non-stick property with respect to toner. It has been desired to provide stripping fingers which solve the above said problems and meet the market requirements such as higher quality, higher reliability and longer life.